The present invention relates to an electric welding pipe joint made of a thermoplastic resin for connecting thermoplastic resin pipes by electric welding, and a method for efficiently producing a thick electric welding pipe joint of a large bore diameter without internal defects.
Conventionally known as a pipe joint for connecting pipes of thermoplastic resins such as polyethylene by electric welding is an electric welding plastic pipe joint disclosed in Japanese Patent Laid-Open No. 57-69010. As shown in FIG. 9, this electric welding plastic pipe joint comprises an inner cylindrical member 31 made of a thermoplastic resin and having grooves 34 extending from one end to the other of the pipe joint, a heating resistance wire 33 wound around the inner cylindrical member 31 in the grooves 34, and an outer cylindrical member 32 made of the same thermoplastic resin as that of the inner cylindrical member 31 integrally molded around the outer surface of the inner cylindrical member 31. By energizing the heating resistance wire 33 embedded in the outer cylindrical member 32 at a boundary with the inner cylindrical member 31, the electric welding pipe joint is integrally fused with the resin pipes inserted into the pipe joint.
The above plastic pipe joint is produced by integrally forming an outer cylindrical thermoplastic resin member 32 around the inner cylindrical member 31 provided with the heating resistance wire 33 by injection molding. Since the heating resistance wire 33 serves to heat the inner surface of the inner cylindrical member 31 and plastic pipes inserted into the pipe joint for heat fusion therebetween, the heating resistance wire 33 should be mounted as close to the inner surface of the inner cylindrical member 31 as possible. For this reason, the inner cylindrical member 31 should have a thickness T.sub.in of 1 mm or less in a flat portion located at the bottom of the grooves 34, even though the pipe joint has a large nominal bore diameter. However, as the nominal bore diameter increases, the thickness of the pipe joint itself increases, leading to an increase in the thickness T.sub.out of the outer cylindrical member 32. In a typical example, a pipe joint for connecting gas pipes made of polyethylene has thicknesses of an inner cylindrical member and an outer cylindrical member at each nominal bore diameter as shown below.
______________________________________ Nominal Bore T.sub.in T.sub.out Diameter (mm) (mm) (mm) ______________________________________ 50 0.5 9 75 0.6 12 100 0.6 14 150 0.6 22 200 0.8 28 ______________________________________
When a thick outer cylindrical member 32 is injection-molded around an outer surface of a thin inner cylindrical member 31 as in the above conventional pipe joint, the inner cylindrical member 31 is melted by large injection pressure and heat provided by the injected outer cylindrical member 32, whereby the heating resistance wire 33 wound around the inner cylindrical member 31 in the groove 34 moves irregularly. As a result, short circuiting takes place between the adjacent heating resistance wires 33 at the time of connecting plastic pipes, failing to achieve desired heat fusion.
The thermoplastic resin generally has an extremely low heat conductivity and suffers from a considerable shrinkage in the course of solidification from a molten state. The shrinkage rate of the thermoplastic resin is as large as 4% to 10%, though it may change depending on the types of thermoplastic resins. Accordingly, some injection molding dies suffer from the following problems: If a resin remains partially unsolidified in a cavity after the solidification of a resin in a gate and a runner, a molten resin cannot be supplied to the partially unsolidified region of the resin in the cavity even though injection pressure is increased, resulting in the formation of voids in a resin layer by shrinkage in the course of solidification.
The voids may be prevented by applying large injection pressure for a long period of time by making the gate and the runner thicker. In this case, however, the thin inner cylindrical member 31 is melted causing the heating resistance wire 33 to move, and it takes long time until a resin is completely solidified in a thick portion T.sub.out, leading to poor production efficiency because of elongated molding cycle.
For the reasons described above, it is difficult to form a thick plastic outer cylindrical member around a thin inner cylindrical member with stable quality by injection molding. This problem is serious particularly in a pipe joint with a nominal bore diameter of 75 mm (3 inches) or more, which comprises an outer cylindrical member having a thickness of 10 mm or more.